1. Field of the Invention
The present invention relates, generally, to a transmission for a motor vehicle that includes a gear actuation system, and more specifically to a transmission having a gear actuation system that electro-mechanically performs both gear selection and gear engagement.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. Presently, there are two typical transmissions widely available for use in conventional motor vehicles. The first and oldest type is the manually operated transmission. These transmissions include a foot-operated start-up or launch clutch that engages and disengages the driveline with the power plant and a gearshift lever to selectively change the gear ratios within the transmission. When driving a vehicle having a manual transmission, the driver must coordinate the operation of the clutch pedal, the gearshift lever, and the accelerator pedal to achieve a smooth and efficient shift from one gear to the next. The structure of a manual transmission is simple and robust and provides good fuel economy by having a direct power connection from the engine to the final drive wheels of the vehicle. Additionally, since the operator is given complete control over the timing of the shifts, the operator is able to dynamically adjust the shifting process so that the vehicle can be driven most efficiently. One disadvantage of the manual transmission is that there is an interruption in the drive connection during gear shifting. This results in losses in efficiency. In addition, there is a great deal of physical interaction required on the part of the operator to shift gears in a vehicle that employs a manual transmission.
The second and newer choice for the transmission of power in a conventional motor vehicle is an automatic transmission. Automatic transmissions offer ease of operation. The driver of a vehicle having an automatic transmission is not required to use both hands, one for the steering wheel and one for the gearshift, and both feet, one for the clutch and one for the accelerator and brake pedal in order to safely operate the vehicle. In addition, an automatic transmission provides greater convenience in stop and go situations, because the driver is not concerned about continuously shifting gears to adjust to the ever-changing speed of traffic. Although conventional automatic transmissions avoid an interruption in the drive connection during gear shifting, they suffer from the disadvantage of reduced efficiency because of the need for hydrokinetic devices, such as torque converters, interposed between the output of the engine and the input of the transmission for transferring kinetic energy therebetween. In addition, automatic transmissions are typically more mechanically complex and therefore more expensive than manual transmissions.
For example, torque converters typically include impeller assemblies that are operatively connected for rotation with the torque input from an internal combustion engine, a turbine assembly that is fluidly connected in driven relationship with the impeller assembly and a stator or reactor assembly. These assemblies together form a substantially toroidal flow passage for kinetic fluid in the torque converter. Each assembly includes a plurality of blades or vanes that act to convert mechanical energy to hydrokinetic energy and back to mechanical energy. The stator assembly of a conventional torque converter is locked against rotation in one direction but is free to spin about an axis in the direction of rotation of the impeller assembly and turbine assembly. When the stator assembly is locked against rotation, the torque is multiplied by the torque converter. During torque multiplication, the output torque is greater than the input torque for the torque converter. However, when there is no torque multiplication, the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. Torque converter slip exists when the speed ratio is less than 1.0 (RPM input>than RPM output of the torque converter). The inherent slip reduces the efficiency of the torque converter.
While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in a parasitic loss, thereby decreasing the efficiency of the entire powertrain. Further, the torque converter itself requires pressurized hydraulic fluid in addition to any pressurized fluid requirements for the actuation of the gear shifting operations. This means that an automatic transmission must have a large capacity pump to provide the necessary hydraulic pressure for both converter engagement and shift changes. The power required to drive the pump and pressurize the fluid introduces additional parasitic losses of efficiency in the automatic transmission.
In an ongoing attempt to provide a vehicle transmission that has the advantages of both types of transmissions with fewer of the drawbacks, combinations of the traditional “manual” and “automatic” transmissions have evolved. Most recently, “automated” variants of conventional manual transmissions have been developed which shift automatically without any input from the vehicle operator. Such automated, or automatic, manual transmissions (AMTs) typically include a plurality of power-operated actuators that are controlled by a transmission controller or some type of electronic control unit (ECU) to automatically shift synchronized clutches that control the engagement of meshed gear wheels traditionally found in manual transmissions. The design variants have included either electrically or hydraulically powered actuators to affect the gear changes. The development of AMTs has provided a viable and improved means of power transmission for motor vehicles over the conventional automatic transmissions having a torque converter. However, even with the inherent improvements of these newer automated transmissions, they still have an operative power interruption in the drive connection between the input shaft and the output shaft during sequential gear shifting. Power interrupted shifting results in a distinct shift feel that is generally associated with manual transmissions and may considered undesirable in certain operating environments.
To eliminate the power interruption in AMTs, other automated manual type transmissions have been developed that can be power-shifted to permit gearshifts to be made under load. Examples of such power-shifted automated manual transmissions are shown in U.S. Pat. No. 5,711,409 issued on Jan. 27, 1998 to Murata for a Twin-Clutch Type Transmission, and U.S. Pat. No. 5,966,989 issued on Apr. 4, 2000 to Reed, Jr. et al for an Electro-mechanical Automatic Transmission having Dual Input Shafts. These particular types of automated manual transmissions have two clutches and are generally referred to simply as dual, or twin, clutch transmissions (DCTs). The dual clutch structure is most often coaxially and cooperatively configured to derive power input from a single engine flywheel arrangement. However, some designs have a dual clutch assembly that is coaxial, but with the clutches located on opposite sides of the transmissions body and having different input sources. Regardless, the layout is the equivalent of having two transmissions in one housing, namely one power transmission assembly on each of two input shafts concomitantly driving one output shaft. Each transmission can be shifted and clutched independently. In this manner, uninterrupted power upshifting and downshifting between gears, along with the high mechanical efficiency of a manual transmission is available in an automatic transmission form. Thus, significant increases in fuel economy and vehicle performance may be achieved through the effective use of certain automated manual transmissions.
The dual clutch transmission structure may include two dry disc clutches each with their own clutch actuator to control the engagement and disengagement of the two-clutch discs independently. While the clutch actuators may be of the electromechanical type, since a lubrication system within the transmission requires a pump, some dual clutch transmissions utilize hydraulic shifting and clutch control. These pumps are most often gerotor types, and are much smaller than those used in conventional automatic transmissions because they typically do not have to supply a torque converter. Thus, any parasitic losses are kept small. Shifts are accomplished by engaging the desired gear prior to a shift event and subsequently engaging the corresponding clutch. With two clutches and two inputs shafts, at certain times, the dual clutch transmission may be in two different gear ratios at once, but only one clutch will be engaged and transmitting power at any given moment. To shift to the next higher gear, first the desired gears on the input shaft of the non-driven clutch assembly are engaged, then the driven clutch is released and the non-driven clutch is engaged.
This requires that the dual clutch transmission be configured to have the forward gear ratios alternatingly arranged on their respective input shafts. In other words, to perform up-shifts from first to second gear, the first and second gears must be on different input shafts. Therefore, the odd gears will be associated with one input shaft and the even gears will be associated with the other input shaft. In view of this convention, the input shafts are generally referred to as the odd and even shafts. Typically, the input shafts transfer the applied torque to a single counter shaft, which includes mating gears to the input shaft gears. The mating gears of the counter shaft are in constant mesh with the gears on the input shafts. The counter shaft also includes an output gear that is meshingly engaged to a gear on the output shaft. Thus, the input torque from the engine is transferred from one of the clutches to an input shaft, through a gear set to the counter shaft and from the counter shaft to the output shaft.
Gear selection and gear engagement in either an AMT or a DCT is similar to that in a conventional manual transmission. One of the gears in each of the gear sets is disposed on its respective shaft in such a manner so that it can freewheel about the shaft. A synchronizer is also disposed on the shaft next to the freewheeling gear so that the synchronizer can selectively engage the gear to the shaft. To automate these transmissions, the mechanical selection of each of the gear sets is typically performed by some type of actuator that moves a shift fork that operatively moves the synchronizers. More specifically, the gear selection and engagement is accomplished by actuating one of a number of shift actuators, with each shift actuator being responsible for engaging a particular synchronizer, and thereby a gear set to its particular shaft. The majority of the newer AMT and DCT designs employ 6 forward gears and a reverse gear, which provides greater efficiency and fuel economy by having closer ratio gear sets than previous designs. However, this also requires between four to seven shift actuators to provide the selective gear engagement. Seven shift actuators are necessary if each gear set employs its own separate shift actuator, and four are necessary if the transmission is designed to employ double-sided, or dual-activating shift actuators that work with paired synchronizers.
Regardless of the exact number, the use of a several separate and generally complex components to provide control over the selective gear actuation in an AMT or DCT is a costly and inefficient approach in automating the relatively simple structure of a manual transmission. With a manual transmission, once the clutch is disengaged, the operator moves a single gear selector into one of a number of positions that mechanically selects the desired gear. The gear selector is operatively connected to one or two shifter rails that each include a number of shift forks that are operatively connected to the synchronizers of the transmission. A set of linkages is sometimes required to couple the gear selector to the shift forks, but the overall design of traditional manual transmissions is relatively simple and efficient. However, the conventional approaches to automating the simple manual transmission structure to provide the current AMT and DCT designs is in sharp contrast to the simplicity of the manual transmission and needlessly complicates the overall structure.
This added-on structural complexity increases manufacturing and assembly costs of the AMT and DCT transmissions. First, each of the separate components must be individually assembled and then separately installed throughout different portions of the transmission. Second, since the shift actuators of this type are most often hydraulic, having a number of these components necessitates the use of the separate hydraulic lines and connections to provide the proper fluid communication between the components and their control devices. When using hydraulic actuators and control devices, inefficiencies also arise from operative performance losses through the hydraulic piping. This also introduces actuation delays and hysteresis to the AMT and DCT operation since the components are not in immediate proximity to each other.
Furthermore, hydraulic components are ultimately electrically controlled by solenoids, such that employing separate hydraulic actuation components requires a number of separate wiring harnesses to electrically connect each electro-hydraulic part, further increasing transmission cost and complexity. The use of separate hydraulic and electrical connections for each separate actuation and control component also increases the likelihood of a failure at these connection points either during manufacture or during the service life of the transmission. The use of some type of non-hydraulic combined or integrated gear selection and engagement system rather than the multi-component conventional gear actuation means for AMTs and DCTs would require many fewer parts, take up less space, be much more cost efficient, and energy efficient.
Accordingly, there exists a need in the related art for a transmission gear actuation system that electro-mechanically performs both gear selection and gear engagement for either an automated manual transmission or a dual clutch transmission application and which replaces many of the previously separate components that are used to control the selective gear engagement.